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D. Petry

Publications and source records attributed to D. Petry.

33 records · Page 2Linked to original sources
arXiv2002

Recent Results from the VERITAS Collaboration

A decade after the discovery of TeV gamma-rays from the blazar Mrk 421 (Punch et al. 1992), the list of TeV blazars has increased to five BL Lac objects: Mrk 421 (Punch et al. 1992; Petry et al. 1996; Piron et al. 2001), Mrk 501 (Quinn et al. 1996; Aharonian et al. 1999; Djannati-Atai et al. 1999), 1ES2344+514 (Catanese et al. 1998), H1426+428 (Horan et al. 2000, 2002; Aharonian et al. 2002; Djannati-Atai et al. 2002) and 1ES1959+650 (Nishiyama et al. 1999; Konopelko et al. 2002; Holder et al. 2002). In this paper we report results from recent observations of Mrk 421, H1426+428 and 1ES1959+650 using the Whipple Observatory 10 m telescope.

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arXiv2002

The TeV spectrum of H1426+428

The BL Lac object H1426+428 was recently detected as a high energy gamma-ray source by the VERITAS collaboration (Horan et al. 2002). We have reanalyzed the 2001 portion of the data used in the detection in order to examine the spectrum of H1426+428 above 250 GeV. We find that the time-averaged spectrum agrees with a power law of the shape dF/dE = 10^(-7.31 +- 0.15(stat) +- 0.16(syst)) x E^(-3.50 +- 0.35(stat) +- 0.05(syst)) m^(-2)s^(-1)TeV^(-1) The statistical evidence from our data for emission above 2.5 TeV is 2.6 sigma. With 95% c.l., the integral flux of H1426+428 above 2.5 TeV is larger than 3% of the corresponding flux from the Crab Nebula. The spectrum is consistent with the (non-contemporaneous) measurement by Aharonian et al. (2002) both in shape and in normalization. Below 800 GeV, the data clearly favours a spectrum steeper than that of any other TeV Blazar observed so far indicating a difference in the processes involved either at the source or in the intervening space.

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arXiv2002

Discovery of Spectral Variability of Markarian 421 at TeV Energies

The detection of spectral variability of the gamma-ray blazar Mrk 421 at TeV energies is reported. Observations with the Whipple Observatory 10m gamma-ray telescope taken in 2000/2001 revealed exceptionally strong and long-lasting flaring activity. Flaring levels of 0.4 to 13 times that of the Crab Nebula flux provided sufficient statistics for a detailed study of the energy spectrum between 380 GeV and 8.2 TeV as a function of flux level. These spectra are well described by a power law with an exponential cutoff. There is no evidence for variation in the cutoff energy with flux, and all spectra are consistent with an average value for the cutoff energy of 4.3 TeV. The spectral index varies between 1.89 in a high flux state and 2.72 in a low state. The correlation between spectral index and flux is tight when averaging over the total 2000/2001 data set. Spectral measurements of Mrk~421 from previous years (1995/96 and 1999) by the Whipple collaboration are consistent with this flux-spectral index correlation, which suggest that this may be a constant or a long-term property of the source. If a similar flux-spectral index correlation were found for other gamma-ray blazars, this universal property could help disentangle the intrinsic emission mechanism from external absorption effects.

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arXiv2001

Large zenith angle observations with the high-resolution GRANITE III camera

The GRANITE III camera of the Whipple Cherenkov Telescope at the Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona (2300 m a.s.l.) has the highest angular resolution of all cameras used on this telescope so far. The central region of the camera has 379 pixels with an individual angular diameter of 0.12 degrees. This makes the instrument especially suitable for observations of gamma-induced air-showers at large zenith angles since the increase in average distance to the shower maximum leads to smaller shower images in the focal plane of the telescope. We examine the performance of the telescope for observations of gamma-induced air-showers at zenith angles up to 63 degrees based on observations of Mkn 421 and using Monte Carlo Simulations. An improvement to the standard data analysis is suggested.

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arXiv2001

Cutoff in the TeV Energy Spectrum of Markarian 421 During Strong Flares in 2001

Exceptionally strong and long lasting flaring activity of the blazar Markarian 421 (Mrk 421) occurred between January and March 2001. Based on the excellent signal-to-noise ratio of the data we derive the energy spectrum between 260 GeV - 17 TeV with unprecedented statistical precision. The spectrum is not well described by a simple power law even with a curvature term. Instead the data can be described by a power law with exponential cutoff: $\rm {{dN}\over{dE}} \propto \rm E^{-2.14 \pm 0.03_{stat}} \times e^{-E/E_{0}} m^{-2} s^{-1} TeV^{-1}$ with $\rm E_{0} = 4.3 \pm 0.3_{stat} TeV$. Mrk 421 is the second $γ$-ray blazar that unambiguously exhibits an absorption-like feature in its spectral energy distribution at 3-6 TeV suggesting that this may be a universal phenomenon, possibly due to the extragalactic infra-red background radiation.

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arXiv2000

Multiwavelength observations of Mkn 501 during the 1997 high state

During the observation period 1997, the nearby Blazar Mkn 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using radio, optical, soft and hard X-ray and TeV data. We concentrate on the medium-timescale variability of the broadband spectra, averaged over weekly intervals. We confirm the previously found correlation between soft and hard X-ray emission and the emission at TeV energies, while the source shows only minor variability at radio and optical wavelengths. The non-linear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet, and flux variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with a Synchrotron Self-Compton (SSC) dominated leptonic jet model, using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet, accounting for gamma-gamma absorption and pair production within the source as well as due to the intergalactic infrared background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found to be negligible at TeV energies. We find that high levels of the X-ray and TeV fluxes can be explained by a hardening of the energy spectra of electrons injected at the base of the jet, in remarkable contrast to the trend found for gamma-ray flares of the flat-spectrum radio quasar PKS 0528+134.

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arXiv1999

The MAGIC Telescope - Prospects for GRB research

The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Telescope collaboration is constructing a large Cherenkov telescope (17 m diameter) for the exploration of the gamma-ray energy regime above 10 GeV with high sensitivity. One of the highlights in the science program of this future observatory are the plans for fast follow-up observations of of GRBs. By ``fast'' we mean delays of less than 30 s between notification and the beginning of observations. The expected gamma counting rates are of the order of 100 Hz for a EGRET counting rate of 0.1 Hz above 100 MeV and a spectral index of 2.0 (this would correspond to a fluence of 10^{-5} erg cm^{-2} for 60 s burst duration). The good photon statistics will permit determination of spectra, search for cutoffs and measurement of light-curves with a time resolution of the order of 1 s.

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arXiv1999

TeV Gamma-ray Observations of the Crab and Mkn 501 during Moonshine and Twilight

TeV Gamma-ray signals from the Crab Nebula and Mkn 501 were detected with the HEGRA CT1 imaging Cerenkov telescope during periods when the moon was shining and during twilight. This was accomplished by lowering the high voltage supply of the photomutipliers in fixed steps up to 13%. No other adjustments were made and no filters were used. Laser runs could not establish any non-linearity in the gain of the individual pixels, and the trigger rate was uniform over the whole camera. The energy threshol was increased by up to a factor of two, depending on the amount of HV reduction. In a series of observations lasting 11.7 hours, a signal with a 3.4 sigma significance was detected from the Crab. During the 1997 multiple flare episode of Mkn 501 a 26 sigma combined excess has been recorded during 134 hours of observations under various moonshine/twilight conditions. The results show that this technique can easily be adapted to increase the exposure of a source, which is important for sources showing rapid time variability such as AGNs or GRBs. Observations can be made up to ~20 deg. angular separation from the moon and until the moon is 85% illuminated (ten to eleven days before and after new moon), as well as during 20 to 40 minutes during twilight, before the commencement of astronomical darkness.

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arXiv1998

Search for gamma-ray bursts above 20 TeV with the HEGRA AIROBICC Cherenkov array

A search for gamma-ray bursts (GRBs) above 20 TeV within the field of view (1 sr) of the HEGRA AIROBICC Cherenkov array (29N, 18W, 2200 m a.s.l.) has been performed using data taken between March 1992 and March 1993. The search is based on an all-sky survey using four time scales, 10 seconds, 1 minute, 4 minutes and 1 hour. No evidence for TeV-emission has been found for the data sample. Flux upper limits are given. A special analysis has been performed for GRBs detected by BATSE and WATCH. Two partially and two fully contained GRBs in our field of view (FOV) were studied. For GRB 920925c which was fully contained in our FOV, the most significant excess has a probability of 7.7 10**-8 (corresponding to 5.4 sigmas) of being caused by a background fluctuation. Correcting this probability with the appropriate trial factor, yields a 99.7% confidence level for this excess to be related to the GRB (corresponding to 2.7 sigmas). This result is discussed within the framework of the WATCH detection.

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arXiv1997

Recent Observations of gamma-rays above 1.5 TeV from Mkn 501 with the HEGRA 5 m^2 Air Cherenkov Telescope

Since February 1997 the BL Lac object Mkn 501 is in a ``high state'' of gamma-ray emission. The HEGRA collaboration has studied Mkn 501 with their air Cherenkov telescopes on La Palma. Here we report on observations with the 5 m^2 telescope (threshold approx. 1.5 TeV) operated in a stand alone mode. We observed a rapidly varying flux between 0.5 to 6 times of that from the Crab Nebula. On average an integral flux above 1.5 TeV of (2 + 1.3 - 0.5) x 10^-11 cm^-2 s^-1 has been determined. The spectrum extends at least up to 10 TeV with an integral power law coefficient of 1.8 +- 0.2 and seems to be steeper than in 1996.

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arXiv1997

AGN sudies above 1.5 TeV with the HEGRA 5 m^2 Cherenkov Telescope (Sources observed: Mkn 421, Mkn 501, MS 0116+319, PKS 2209+236, NGC 315 and W Comae)

The HEGRA 5 m^2 air Cherenkov telescope (CT1) was used to search for gamma-ray emission above 1.5 TeV from a series of low redshift AGN (Mkn 421, Mkn 501, MS 0116+319, PKS 2209+236, NGC 315 and W Comae). Here we present results from a total of 383 hours of ON-source observations at zenith angles up to 35$^\circ$ made between February 1996 and April 1997 showing positive detections of Mkn 421 and Mkn 501 and upper limits on the other objects' emission. More recent results from observations of Mkn 421 und Mkn 501 will be added at the conference.

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arXiv1997

A Multi-frequency campaign on the gamma-ray loud blazar W Comae

We report preliminary results of a multi-frequency campaign on the TeV candidate blazar W Comae (z=0.102). Flux limits by Whipple and HEGRA show that the TeV flux must be considerably below an E^-2 extrapolation of the EGRET flux. In the framework of proton-initiated cascade models, this implies a moderate amount of gamma-ray attenuation due to pair production in collisions with low-energy photons of the extragalactic infrared background. In a simple SSC model, the gamma-ray spectrum cuts off intrinsically just below TeV without requiring external absorption.

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arXiv1997

Detection of Gamma-Rays above 1.5 TeV from Mkn 501

A detection of TeV $γ$-rays from Mkn 501 is reported, based on observations made between March and August 1996 with the first HEGRA Cherenkov telescope (CT1). From the image analysis, 351 excess candidate $γ$-ray events are obtained from the 147 h dataset. The statistical significance of the excess is 5.2 $σ$. The average excess rate is $2.4 \pm 0.5$ h$^{-1}$ above the $\approx$ 1.5 TeV threshold of CT1. Under the assumption that the spectrum of Mkn 501 follows a power law we find a differential spectral index of 2.6$\pm0.5$ and obtain a time-averaged integral flux above 1.5 TeV of $2.3 (\pm0.4)_{Stat} (+1.5-0.6)_{Syst} \times 10^{-12}$ cm$^{-2}$s$^{-1}$. Comparison with our near contemporary observations of the Crab Nebula, used as a standard candle to test CT1 after upgrading to a 127 pixel camera, indicates that Mkn 501 has a spectrum similar to that of the Crab Nebula above 1.5 TeV. The integral flux above 1.5 TeV from Mkn 501 is found to have been between 2.2 and 3.6 times smaller than that from the Crab Nebula. HEGRA is the second experiment to have detected Mkn 501 in the TeV range.

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arXiv1996

Detection of VHE Gamma-rays from Mkn 421 with the HEGRA Cherenkov Telescopes

A detection of Gamma-rays from Mkn 421 at energies above 1 TeV is reported, based on observations made in December 1994 - May 1995 with the first two HEGRA Cherenkov telescopes. >From the image analysis, 111 excess gamma candidates are obtained from the 26 h Telescope #1 (CT1) dataset (significance 4.0 sigma) and 218 from the 41 h Telescope #2 (CT2) dataset (significance 4.2 sigma) at zenith angles theta < 25 degrees. The combined significance is approx. 5.8 sigma. This is the second detection of Mkn 421 at TeV energies. The average excess rate is 4.3 +- 1.0 h^-1 for CT1 and 5.4 +- 1.3 h^-1 for CT2. Comparison with our contemporary observations of the Crab Nebula indicates that Mkn 421 has a steeper spectrum than the Crab Nebula above 1 TeV. Under the assumption that the spectrum of Mkn 421 follows a power law, we obtain a differential spectral index of 3.6 +- 1.0 and an integral flux above 1 TeV of 8 (+-2)_(Stat) (+6-3)_(Syst) x 10^-12 cm^-2 s^-1 from a comparison with Monte Carlo data. This flux is smaller than the Crab Nebula flux by a factor of 2.0 +- 0.8.

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